Presently available medical apparatuses for capturing diagnostic images of patients include CT (Computed Tomography) scanners, MRI (Magnetic Resonance Imaging) apparatuses, PET (Positron Emission Tomography) apparatuses, ultrasonic diagnostic apparatuses and the like. Angiography apparatuses, MRA (MR angiography) apparatuses and the like are currently used as medical apparatuses for capturing vascular images of patients.
When the abovementioned medical apparatuses are used, a liquid such as a contrast medium or physiological saline may be injected into a patient. Chemical liquid injectors for automatically performing the injection have been put into practical use. A liquid syringe, for example including a cylinder member and a piston member slidably inserted into the cylinder member, is mounted on such a chemical liquid injector. A syringe driving mechanism presses the piston member into the cylinder member. The cylinder member is filled with a liquid and connected to a blood vessel of a human body near the surface thereof through an extension tube and an injection needle. Thus, the liquid in the liquid syringe is injected with pressure into the blood vessel of the human body by the chemical liquid injector.
Such a chemical liquid injector automatically injects the liquid at high pressure. For example, when the injection needle deviates from the vessel to leak under skin, an operator does not easily recognize it immediately. To solve the problem, various leak detecting apparatuses have been proposed for detecting leak of a liquid injected into a blood vessel of a human body near the surface thereof through an injection needle (see, for example, patent documents 1 to 8 below).    Patent document 1: U.S. Pat. No. 6,408,204    Patent Document 2: U.S. Pat. No. 5,964,703    Patent Document 3: U.S. Pat. No. 5,947,910    Patent Document 4: U.S. Pat. No. 6,375,624    Patent Document 5: U.S. Pat. No. 5,954,668    Patent Document 6: U.S. Pat. No. 5,334,141    Patent Document 7: U.S. Pat. No. 4,647,281    Patent Document 8: U.S. Pat. No. 4,877,034